A major aim of our work is the definition, in precise molecular terms, of the function of E. coli DNA ligase, E. coli DNA polymerase I, T4 DNA ligase and T4 DNA polymerase in DNA replication. A second aim is the identification and characterization of as yet unknown enzymes and factors involved in DNA replication. Our specific goals for the coming year are the following: a. Resolution of the question of whether post replication repair of uracil in DNA contributes to the pool of Okazaki fragments seen in (3H) thymidine experiments. b. Accumulation of several hundred milligrams of homogeneous E. coli DNA ligase; crystallization of the enzyme and initiation of a detailed study of its structure and mechanism. c. Beginning of a study of the DNA polymerase of Drosophila melanogaster. This new venture represents the beginning of an attempt to understand the mechanism of DNA replication in a relatively simple eukaryote.